Stroke volume and cardiac output are measures of the performance of a heart, and thus are very important to physicians. Stroke volume is the volume of blood pumped out of the heart in one beat. Cardiac output is the volume of blood pumped out of the heart in one minute. Physicians may use stroke volume and cardiac output measurements for evaluating and monitoring cardiac function in patients with congestive heart failure or other cardiovascular maladies. Stroke volume and cardiac output are also used for diagnosis and to guide therapeutic decisions.
One conventional technique for estimating cardiac output is sometimes referred to as the Fick method. This technique involves having a patient breathe pure O2, and measuring the O2 uptake directly from the net blood gas flux and measuring the blood oxygen content via venous and arterial blood samples. The blood samples can be taken with two catheters, one located in an artery, such as the brachial artery, and another located in a vein, such as the femoral vein. While this method may yield an accurate estimation of cardiac output, the disadvantages of this method include an inability to make continuous real-time measurements, and that the measurements necessarily occur in a hospital or clinic. Additionally, the measurements may not be continuously repeated due to the invasiveness of the method and the significant amount of blood required for each measurement.
Another existing method for measuring cardiac output is the indicator dilution or thermo-dilution technique, which involves the injection of a predetermined amount of an indicator dye or relatively cool saline solution into the right atrium. The dilution of the dye or the temperature and conductivity of the saline is measured downstream. The average volume of blood flow is inversely proportional to the integrated area under the measured dilution curve. The main disadvantage of indicator dilution is that it does not allow continuous estimation of cardiac output because recirculation of dye through the bloodstream corrupts subsequent measurements. Thermo-dilutions also may not be repeated for a long period of time because it dilutes the patient's blood supply. Additionally, these methods provide only average values and occur only in a hospital or clinic setting.
One commonly used existing method for continuously estimating stroke volume and cardiac output involves the use of echocardiography. Using echo-Doppler ultrasound equipment, the velocity of the blood as it travels through an outflow tract during a heartbeat may be measured. These velocity measurements form velocity-time curves. Stroke volume may be continuously estimated as a function of the results of integration of measured velocity-time curves. Cardiac output may be calculated using the estimated stroke volume value. Although echocardiography produces an accurate estimation of stroke volume and cardiac output, this method requires bulky ultrasound equipment that restricts its use to hospitals and clinics.
Other continuous estimation methods include techniques involving the use of impedance measurements to detect changes in blood volume, and techniques involving acute measurement of the blood flow from the heart with a flow probe. Neither of these methods, nor any of the above-discussed methods, allows for continuous estimation of stroke volume or cardiac output in an outpatient setting via an implantable device. Because the condition of a patient may change between visits to a physician, and because these changes in condition between visits may be of use to the physician when he or she is making future therapeutic decisions, it is desirable to monitor the patient's stroke volume or cardiac output continuously in an outpatient setting.
One existing method for continuously estimating cardiac output as a function of pressure measured in a heart is disclosed in U.S. Pat. No. 5,797,395, issued to Martin. The method disclosed in the Martin patent involves the measurement of an aortic pressure wave with a catheter inserted in the radial artery. The characteristics of the measured pressure wave are then compared to characteristics of pressure waves with known cardiac output values to determine a match and to thus determine a cardiac output value for the measured pressure wave. The disadvantages of this method are that it is not applicable to an outpatient setting, and that it requires substantial and complex processing in order to yield an accurate result.
Because these changes in condition may reflect or lead to a sudden deterioration of the patient's condition, particularly in the case of patients with congestive heart failure, it may be desirable to continuously adjust a therapy in an outpatient setting as a function of the estimated stroke volume or cardiac output. These adjustments to the therapy may avert deterioration in condition between office visits, and thus the patient may avoid a possible hospitalization or death. One existing method for continuously estimating cardiac output in an outpatient setting as a function of pressure measured in a heart, and adjusting cardiac pacing parameters as a function of the estimated cardiac output is disclosed in U.S. Pat. No. 6,314,323 B1, issued to Ekwall. The Ekwall patent discloses estimating cardiac output by integrating a ventricular pressure curve between a time of valve opening and a time of valve closing. A disadvantage of this method is that it requires complex processing of the pressure signal in order to estimate cardiac output. Thus, the complexity, expense, and power consumption of an implantable device that utilizes this method to estimate cardiac output may be increased.
Examples of the above referenced existing techniques and/or devices for determining stroke volume and cardiac output, and existing techniques for measuring the pressure of blood in a heart may be found in the issued U.S. Patents listed in Table 1 below.
TABLE 1Patent No.InventorIssue Date6,325,762TjinDec. 04, 20016,314,323 B1EkwallNov. 06, 20016,217,522 B1ShoshanApr. 17, 2001WO 00/51495Buck et al.Sep. 08, 20005,868,676McCabe et al.Feb. 09, 19995,810,735Halperin et al.Sep. 22, 19985,797,395MartinAug. 25, 19985,626,623Kievel et al.May 06, 19975,606,972RouthMar. 04, 19975,535,752Halperin et al.Jul. 16, 19965,368,040CarneyNov. 29, 19945,287,753Routh et al.Feb. 22, 1994
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the techniques of the present invention.